Help a Newbie with his Magellan Meridian

[+GPS Joe]

I am having problems with my Meridian Color giving me different coordinates when I try to save a waypoint when hiding a cache. Yesterday I hid a cache and came back to it today to check my coordinates and it was off by 100'. I did it again and it was off by 30'. Am I doing something wrong?

 

I notice that my screen will sometimes say EPE 26ft, or EPE 56ft or WAAS. What do these mean? The maunal is not very clear.

 

Thanks to all who reply.

[greatwhite6]

Check the software version shown on the bottom of the start-up screen. If you are not at ver. 4.02 then download it from this URL http://www.magellangps.com/en/support/products/upgrades/upgrade_meridiangps.asp

This should improve the unit. I downloaded it the day I brought my unit home and it's locked right on since day one.

 

"Faith is to the human what sand is to the ostrich"

 

><DARWIN>

L L

[+headmj]

Another tip to try is that when you are setting the cache try to get a good average on the location. 10 minutes should be enough for teh Meridian to come down to about 10 ft epe. the WAAS averaging for 10 minutes is what you want. Markwell has a good set of checklists on his website for setting caches.

[+GPS Joe]

I have 4.02 "WAAS ENABLED", whatever that means.

 

Thanks.

[+blindleader]

quote:

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and it was off by 100'.

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Welcome to the world of measurement. It wasn't "off" by 100’; It disagreed with the previous fix by 100'. The difference is important. To get accurate fixes when placing a cache you need to learn to recognize a good fix (low EPE, receiving WAAS) from a bad one (high EPE, no WAAS).

 

Nobody but Magellan knows what EPE means. It stands for Estimated Position Error, and describes the circle within which the unit thinks there is some percentage of probability that the true fix is based on the unit's calculation of whatever DOP (Dilution Of Position) number it uses. I'm unaware of any manufacturer of consumer GPSRs disclosing what that percentage is, what algorithm is used to calculate it, or much of anything else useful about how their units operate.

 

WAAS stands for Wide Area Augmentation System. It’s the FAA’s system of broadcasting atmospheric correction data to WAAS enabled GPSRs. Normally, it greatly improves the accuracy of a fix.

 

I don’t put much stock in autoaveraging. You just need to make sure a Meridian sits still long enough to settle down. Ten seconds is plenty if you’ve been within a few yards of the cache for a few minutes. See below for more.

 

[Nerd alert – Stop reading if you are offended by nerdity or if you are subject to seizures when exposed to technical subjects.]

 

From my own experience with My Merigreen, I believe Magellan uses a 95% figure for the EPE calculation. That is, if you have an EPE of 26 feet then there is a 95% probability that you are within 26 feet of the coordinates shown. Another thing I sort of believe is that it uses more than four satellites, if available, to over-determine the fix.

 

If you’re really anal retentive and into computers and such gadgets, you can further refine how “good” a fix is by recording the exact time of the fix, and list the numbers of the satellites being received at that moment. Then you take that information and plug it into a Satellite Availability Program to calculate GDOP for your fix. There are numerous threads mentioning this software, mostly in the GPS Units and Software forum, and some of them should help. My own personal technique is that I average multiple single fixes over several days, at different times of the day, calculating GDOP for every one of them. I never use a fix with a GDOP over four. If exposure to sky is good then <2.5 is the standard I use.

 

The reason I don’t care for autoaveraging is that if you’re there at a bad time of day, a ten minute average gives you the average of 600 crappy fixes, which adds up to one crappy fix. If you have a decent fix to begin with, then the average showing after ten minutes will be the same as the initial ten seconds and you’ve just wasted ten minutes that you could be wasting on internet forums. The reason 600 fixes over ten minutes don’t average to a good fix is that the atmospheric errors (the largest component of error for consumer GPSRs) do not change over such a short period. The satellites all are in nearly same spots and the atmosphere along the line from each Satellite to your receiver is pretty much the same as it was ten minutes ago.